1. Field of the Invention
The invention concerns a device for drawing an optical fiber from a preform, comprising a furnace for melting one end of the preform, from which the optical fiber is drawn, and an optical sensor at the exit from the furnace for measuring the diameter of the optical fiber for the purpose of adjusting the drawing speed and tension.
2. Description of the Prior Art
In a device of the above kind the preform is generally made of silica and is moved slowly in translation inside the furnace. The Latter provides the energy needed to melt a drawing cone at one end of the preform from which the optical fiber is drawn by drawing means.
The diameter of the optical fiber depends on the drawing speed and the drawing tension applied to the optical fiber by the drawing means. In an arrangement known in itself the optical sensor is disposed at the exit from the furnace to monitor the formation of the optical fiber and to adjust the drawing speed and the drawing tension rapidly to guarantee a diameter inside the range of tolerances.
If the optical sensor detects a fiber diameter outside the range of tolerance, indicating a defect, the optical fiber is cut on opposite sides of the measurement point. At present an optical fiber must have over a length of 2.2 kilometers (km) a minimum tolerance in respect of the diameter of plus or minus 2 microns (xcexcm) for a mean diameter of 125 xcexcm. Future requirements will be for a minimal commercial length of 5 km and a tolerance of plus or minus 1 xcexcm.
The optical sensor measures the diameter using a principle known in itself and consisting in illuminating the optical fiber by means of a light source and measuring the shadow projected by the diameter of the optical fiber onto a photodiode receiver. The sensor therefore has the advantage of a measurement point with no mechanical contact. The shadow is generated dynamically by scanning a laser beam to and fro across the optical fiber at a frequency of 1 000 Hertz (Hz) for example. The measured diameter is therefore free of any uncertainty due to vibration of the optical fiber between the furnace and the drawing means, at a frequency typically less than 10 Hz.
The measurement accuracy of the optical sensor must be related to the range of tolerances so as not to cause the fiber to be cut as a consequence of measurement artifacts. Tests carried out on site using a device of the type just described showed that the accuracy of measurement of the diameter of the optical fiber as measured during drawing was incompatible with the drawing process, being some five times worse than the accuracy achieved in tests carried out off-site with the same optical sensor and samples of the same optical fiber. The expression xe2x80x9coff-sitexe2x80x9d means with the optical sensor disposed away from the fiber drawing device, the samples being immobile in front of the optical sensor.
The aim of the invention is to improve the measurement accuracy of an optical sensor near the furnace of a drawing device.
To this end, the invention consists in a device for drawing an optical fiber from a preform, comprising a furnace for melting one end of the preform from which the optical fiber is drawn and an optical sensor disposed at the exit from the furnace to measure the diameter of the optical fiber in order to adjust the drawing speed and the drawing tension, wherein the optical sensor is surrounded by a mask forming a dark chamber through which the optical fiber passes.
Other features and advantages of the invention will become apparent upon reading the description of one embodiment of the invention illustrated by the drawings.